The present invention relates to a method and a device for representing a composite image on a screen of a screen device, said image being composed of a plurality of natural and/or synthetic sub-images which are generated by different sources.
The representation of an image on the screen of a screen device is provided by a plurality of lines, e.g., 625 lines, which are scanned by an electron beam. When the electron beam has scanned all the lines of the screen, the entire process is repeated. In order to scan the lines in a regular manner, the electron beam must be synchronized in the horizontal and the vertical direction. The electron beam is clocked from one line to the following line by the clock for the horizontal synchronization, the so-called line clock, while said beam returns to the starting point of the image at the clock for the vertical synchronization, the so-called image clock, to again scan line after line of the screen.
Screen devices for representation of characters and graphics in addition comprise, contrary to common TV-devices, a further clock besides the line clock and the image clock. At the clock, the so-called dot clock or pixel clock, each line is split up into a plurality of pixels. The electron beam in this case continuously moves along the line such as in common TV-devices, but its black-white control is clocked by the pixel clock whereby the resolution of the pixels is determined by the period of the pixel clock.
This common principle can be used to fade in determined points of the screen color- or image-halftones not provided by the associated image source, e.g., a TV-camera. Methods for fading graphical elements in the image of a screen device are already known (DE-OS 34 10 662). Furthermore, techniques such as "image in image" an analogue studio technique or "window technique" in screen edited computer working places using totally synthetic raster images are known in the art (German Pat. No. 30 35 636).
In a display apparatus having a color graphic display it is known to make use of three (blue, red and green) bit map memory planes and to perform logic operations within the respective memory planes for extracting or suppressing a specific color (U.S. Pat. No. 4,613,852, granted to Kinya Maruko on Sept. 23, 1986).
It is an object of the invention to provide a method and a device of the mentioned kind providing an improved composite image representation on a screen of at least two sub-images generated by different sources.
The method and the device according to the invention provide a representation of images particularly on high resolution screens, which images may be composed of sub-images from different sources by keeping up with the pixel frequency of several 10 MHz, from pixel to pixel, from line to line and from sub-image to sub-image. The picture sources may be driven asynchronously and with different image generating frequencies.
Other objects and advantages of the invention are mentioned and explained in detail in the following description when taken together with the accompanying drawing.